Purpose: Linear accelerators have introduced delivery control systems with the ability to deliver Volumetric Modulated Arc therapy (VMAT) either by a single arc or by multiple-arc. VMAT delivery for complex treatments such as head and neck requires providing more number of segments for a given plan. The primary goal of this study is to combine these multiple arcs into a single arc and compare the dose distribution and the treatment time for the two plans.

Methods: Five Head and Neck plans and five prostate plans were considered for this study. VMAT plans were generated using Varian SmartArc treatment planning system version 8.5. A combined DICOM plan with single arc is created which has half the spacing as the original multiple arc plan and have all the segments of the multiple arc plan. For the combined arc plan and the multiple arc plans, the delivery time for optimal delivery under delivery constraints is determined using the delivery algorithm. A fluence map comparison between the combined arc and multiple arcs is made to prove the dosimetric similarity of the arc plans.

Results: The delivery time for the Head & Neck plans for the combined arc is improved by 35% and for the prostate plan is improved by 37% as compared to the multiple arc plans. Difference fluence is calculated from the multiple arc fluence and the combined arc fluence for all the plans. There is less than 5% dosimetric difference between the multiple arc plans and the combined arc plans for both prostate and head and neck plans.

Conclusion: Combined arc delivery provides comparable plan quality and improved treatment delivery time. Hence, for more complex sites combined arc delivery is preferable due to the improvement in the treatment time while maintaining similar dose distribution with that of the multiple arc plans.